Method for producing grain oriented magnetic steel sheet and grain oriented magnetic steel sheet

ABSTRACT

In a method for manufacturing a grain-oriented electrical steel sheet using steel containing less than 100 ppm of Al and 50 ppm or less each of N, S, and Se as a starting material, purification annealing is performed at 1050° C. or more, the partial pressure of hydrogen in the atmosphere being adjusted to 0.4 atm or less in a temperature range above 1170° C. for a purification annealing conducted at a temperature above 1170° C., or 0.8 atm or less in a temperature range of 1050° C. or more for a purification annealing conducted at a temperature of 1170° C. or less, to prevent deterioration of the bend properties due to the impurities.

TECHNICAL FIELD

This disclosure relates to a grain-oriented electrical steel sheet withexcellent magnetic and bend properties, and to a method formanufacturing the grain-oriented electrical steel sheet consistently. Inparticular, the disclosure provides an advantageous effect when a steelsheet is, but not limited to, strip-shaped or a steel strip.

BACKGROUND ART Prior Art

In manufacturing a grain-oriented electrical steel sheet, a precipitatethat is known as an inhibitor is generally used for preferentialsecondary-recrystallization of {110} <001>-oriented grain, which iscalled Goss-oriented grain during finishing-annealing.

For example, methods in which MnS or MnSe (Japanese Examined PatentApplication Publication No. 51-13469) and AIN are used as inhibitorshave already been put to practical use. Furthermore, BN and nitrides ofTi, Zr, and V are also known as inhibitors.

In conventional methods as described in Japanese Examined PatentApplication Publication No. 51-13469, finishing-annealing typicallyincludes secondary-recrystallization annealing and subsequentpurification annealing for the purpose of film formation andpurification.

While the secondary-recrystallization annealing can be performed invarious atmospheres, it is believed that nitrogen-containing atmospheresare most suitable to stabilize the behavior of effective inhibitors,such as nitrides.

On the other hand, the purification annealing is typically performed inhydrogen-based atmospheres, preferably in a hydrogen atmosphere toenhance the removal of impurities in the steel, such as an inhibitor. Inparticular, a nitrogen as a component of the atmosphere is notpreferred, because a high nitrogen content results in insufficientremoval of nitrogen in the steel, and therefore little improvement inthe magnetic property of the steel sheet can be achieved. For example,Japanese Unexamined Patent Application Publication No. 11-158557describes the adverse effect of a nitrogen atmosphere (about 0.1-0.4atm) in the purification annealing.

In general, the purification annealing is preferably performed at 1180°C. or more. The purification annealing below 1180° C. results ininsufficient removal of impurities in the steel, such as S and Se, andleads to inferior bend properties of the steel sheet.

The bend properties are evaluated by a repeated bending test inaccordance with JIS C 2550; a specimen 30 mm in width that is cut from asteel sheet is repeatedly bent at right angles under tension and thenumber of bendings is counted until a crack penetrates through thespecimen in the thickness direction.

Although methods including the use of inhibitors are useful toconsistently develop secondary-recrystallization grain, they requirefine dispersion of precipitates and thus a slab must be heated to atleast 1300° C. before hot rolling.

However, heating the slab to such a high temperature disadvantageously(1) increases equipment cost, (2) reduces yields owing to an increasedamount of scale during hot rolling, and (3) complicates maintenance offacilities.

In contrast to these methods, methods for manufacturing a grain-orientedelectrical steel sheet without using inhibitors are disclosed inJapanese Unexamined Patent Application Publication Nos. 64-55339,2-57635 and 7-197126.

All the methods in Japanese Unexamined Patent Application PublicationNos. 64-55339, 2-57635 and 7-197126 preferentially develop a {1 110}surface by using surface energy as a driving force. Thus, impurities inthe steel sheet are reduced in advance and then finishing-annealing athigh temperature is performed in a controlled atmosphere to prevent thegeneration of surface oxides to enhance secondary-recrystallization.

For example, Japanese Unexamined Patent Application Publication No.64-55339 describes a technique for preparing an integratedrecrystallized structure with {110} <001> orientation, in which asilicon steel sheet prepared by melting highly purified raw materials,such as electrolytic iron, is rolled to a thickness of 0.2 mm or less,and is then heat-treated at 1180° C. or more in vacuo or in anatmosphere of an inert gas, hydrogen, or a mixture of hydrogen andnitrogen.

Japanese Unexamined Patent Application Publication No. 2-57635 describesa technique in which a commercial silicon steel strip is coated with anannealing separator to remove impurities, such as AIN and MnS, ispurified at 1100-1200° C. under a hydrogen atmosphere for 3 hours ormore, is cold-rolled to a thickness of 0.15 mm or less, and is thensubjected to secondary-recrystallization annealing at 950-1100° C. in anatmosphere of an inert gas such as Ar, hydrogen, or a mixture ofhydrogen and an inert gas, and preferably under reduced pressure.

In Japanese Unexamined Patent Application Publication No. 7-197126,silicon steel in which S, an impurity having a particularly largeadverse effect, is reduced to 10 ppm, is subjected to short-timefinishing-annealing at 1000-1300° C. in a nonoxidative atmosphere withan oxygen partial pressure of 0.5 Pa or less, or in vacuo for 10 minutesor less.

These techniques do not place importance on purification annealing aftersecondary-recrystallization and do not particularly disclose it.

The above-mentioned manufacturing processes that utilize surface energydo not require as high a temperature as the conventional methods used toheat the slab, but they have the following problems:

First, for effective use of the surface energy difference, the thicknessof a steel sheet must be small to increase the contribution of thesurface. For example, in the techniques disclosed in Japanese UnexaminedPatent Application Publication Nos. 64-55339 and 2-57635 the thicknessesof the steel sheets are limited to not more than 0.2 mm and 0.15 mm,respectively.

However, most of the currently-used grain-oriented electrical steelsheets are 0.20 mm or more in thickness, and thus it is difficult tomanufacture a grain-oriented electrical steel sheet with excellentmagnetic properties using the surface energy.

Second, as described above, an atmosphere of an inert gas or hydrogen,and preferably a vacuum is required for finishing-annealing for thesecondary-recrystallization. However, the combination of hightemperature and a vacuum is very difficult to achieve and expensive infacilities.

Third, the use of the surface energy, in principle, only allows for theselection of a {110} surface, and does not necessarily allow for thedevelopment of <001>-oriented Goss grain along a rolling direction.

Since the magnetic property of the grain-oriented electrical steel sheetcan be improved only when the axis of easy magnetization <001> isoriented toward the rolling direction, selection of only the {110}surface, in principle, does not provide a satisfactory magneticproperty. Thus, the rolling and annealing conditions which can achieveexcellent magnetic properties in methods utilizing the surface energyare limited and the resulting magnetic properties will most likely beunstable.

Fourth, of the methods utilizing the surface energy, thefinishing-annealing must be performed while inhibiting the formation ofa surface oxide layer, and thus cannot be performed when an annealingseparator is applied to a steel sheet. Thus, unlike typicalgrain-oriented electrical steel sheets, an oxide film cannot be formedafter the finishing-annealing. A forsterite film, which is formed when aMgO-based annealing separator is applied to the steel sheet, forexample, generates tension on the surface of the steel sheet to improveiron loss. In addition, phosphate-based insulating tension-coating onthe forsterite film ensures adhesion of the coating and further improvesiron loss. Therefore, the absence of a forsterite film on the steelsheet results in poor adhesion between the tension-coating and the steelsheet, and the iron loss increases significantly.

Under these circumstances, in Japanese Unexamined Patent ApplicationPublication Nos. 2000-129356 and 2000-119824, we proposed techniques fordeveloping a Goss-oriented crystal grain duringsecondary-recrystallization of materials that do not contain aninhibitor by controlling the difference in the grain boundary mobility(details are shown below). Using these techniques, crystal grain can bebrought into Goss orientation without using surface energy, thusovercoming the problems described above. For example, since thesetechniques are not limited by the surface condition of the steel sheet,an annealing separator can be applied to the steel sheet beforefinishing-annealing to form a film, such as a forsterite film, andthereby iron loss can be improved. For convenience, the grain-orientedelectrical steel sheet proposed in Japanese Unexamined PatentApplication Publication No. 2000-129356 and the like is hereinafterreferred to as inhibitor-free steel sheet.

In the technique proposed in Japanese Unexamined Patent ApplicationPublication No. 2000-129356 and soon, since the Al content is reduced toa predetermined level and the S and Se contents are also limited,conventional purification annealing is not necessarily required and thesteel sheet is simply heated to a temperature at which a film, such as aforsterite film, forms after the secondary-recrystallization annealing.For example, Japanese Unexamined Patent Application Publication No.2000-129356 shows a finishing-annealing condition in which annealing iscompleted by heating the steel sheet to about 950-1050° C. at a rate ofabout 15-20° C./h in a nitrogen atmosphere or nitrogen-containingatmosphere.

However, purification anncaling is not necessarily precluded in thetechnique, and purification annealing that allows for further reductionof impurities in the steel is rather effective in further improving themagnetic properties. For example, Japanese Unexamined Patent ApplicationPublication No. 2000-119824 discloses a technique in which thefinishing-annealing is performed by heating the steel sheet to 1180° C.in a mixed atmosphere of 50% hydrogen and 50% nitrogen, and then bykeeping the steel sheet at 1180° C. for 5 hours in a hydrogenatmosphere. Even if purification annealing is performed, the absence ofinhibitors results in a reduced operating load. For example,purification annealing at a lower temperature can achieve a sufficienteffect.

Furthermore, in some techniques, secondary-recrystallization annealingand purification annealing are indistinguishable from each other. Forexample, Japanese Unexamined Patent Application Publication No.2000-119824 discloses a technique in which the finishing-annealing isperformed by increasing the temperature to about 1100° C. at a rate ofabout 20° C./h in a mixed atmosphere of 50% hydrogen and 50% nitrogen,or by increasing the temperature to 1200° C. at a rate of 15° C./h in ahydrogen atmosphere.

Japanese Unexamined Patent Application Publication No. 2000-119823describes a technique in which finishing-annealing is performed usingsteel that is free of inhibitors at about 1000-1150° C. in an atmosphereof, for example, nitrogen, Ar, hydrogen, 50% hydrogen and 50% nitrogen,50% nitrogen and 50% Ar.

As described above, when impurities in steel, such as S and Se, areinsufficiently removed, the bend properties will deteriorate. In aninhibitor-free steel sheet, the contents of S and Se after purificationannealing should be low enough so as not to affect the bend properties.Nevertheless, it became apparent that a final sheet product ofinhibitor-free steel might have deteriorated bend properties. Thus, thisindicates the presence of another cause of deterioration in the bendproperties, other than the insufficient removal of S and Se.

Poor bend properties may result in the fracture of the steel sheet in apunching line or the generation of cracks in the steel sheet in theproduction of a wound-core transformer. These problems may occur evenwhen, for example, only a portion of an electrical steel strip in thetransverse direction (for example, transverse end) has deteriorated bendproperties.

Accordingly, it could be advantageous to improve the technique formanufacturing a grain-oriented electrical steel sheet without usinginhibitors (inhibitor-free steel sheet) as disclosed in JapaneseUnexamined Patent Application Publication No. 2000-129356 and the liketo prevent deterioration in the bend properties.

SUMMARY

We provide the following aspects:

(1) A method for manufacturing a grain-oriented electrical steel sheetwith excellent bend properties, comprising the steps of:

rolling a steel slab containing 0.08 mass percent or less of carbon,2.0-8.0 mass percent of Si, and 0.005-3.0 mass percent of Mn into acold-rolled steel sheet;

subsequently performing decarburizing annealing of the cold-rolled steelsheet if desired;

subsequently applying an annealing separator to the cold-rolled steelsheet if desired;

performing secondary-recrystallization annealing of the cold-rolledsteel sheet; and

subsequently performing purification annealing of the cold-rolled steelsheet,

wherein the steel slab contains less than 100 ppm of Al and not morethan 50 ppm each of N, S, and Se, the purification annealing isperformed at 1050° C. or more, and the partial pressure of hydrogen inthe atmosphere is adjusted to 0.4 atm or less in a temperature rangeabove 1170° C. for a purification annealing conducted at a temperatureabove 1170° C., or 0.8 atm or less in a temperature range of 1050° C. ormore for a purification annealing conducted at a temperature of 1170° C.or less.

Preferably, the annealing separator is a MgO-based annealing separator.

Preferably, the rolling step includes the substeps of hot-rolling thesteel slab, annealing the hot-rolled steel sheet if desired, performingcold-rolling one time, or at least two times with intermediate annealingtherebetween to produce the cold-rolled steel sheet.

Preferably, in the purification annealing, nitrogen in the atmosphere inwhich the hydrogen partial pressure is controlled is less than 50% byvolume fraction.

(2) The method for manufacturing a grain-oriented electrical steel sheetwith excellent bend properties according to aspect (1) and its preferredembodiment, wherein the steel slab further contains 0.005-1.50 masspercent of Ni and/or 0.01-1.50 mass percent of Cu.

(3) The method for manufacturing a grain-oriented electrical steel sheetwith excellent bend properties according to aspect (1) or (2) and itspreferred embodiments, wherein the steel slab further contains a totalof 0.0050-0.50 mass percent of at least one of Cr, As, Te, Sb, Sn, P,Bi, Hg, Pb, Zn, and Cd, and the partial pressure of the hydrogenatmosphere is adjusted to 0.2 atm or less in a temperature range above1170° C. for a purification annealing conducted at a temperature above1170° C., or 0.6 atm or less in a temperature range of 1050° C. or morefor a purification annealing conducted at a temperature of 1170° C. orless.

Preferably, the steel slab contains at least one of As, Te, Sb, Sn, P,Bi, Hg, Pb, Zn, and Cd.

(4) The method for manufacturing a grain-oriented electrical steel sheetwith excellent bend properties according to any of aspects (1) to (3)and their preferred embodiments, and a strip-shaped grain-orientedelectrical steel sheet (or a grain-oriented electrical steel strip)manufactured by the method, wherein the rolling includes a cold-rollingsubstep of preparing a cold-rolled steel strip, and the cold-rolledsteel strip is subjected to secondary-recrystallization annealing andpurification annealing to produce a strip-shaped grain-orientedelectrical steel sheet.

(5) A strip-shaped grain-oriented electrical steel sheet containing2.0-8.0 mass percent of Si, 0.005-3.0 mass percent of Mn, and 35 ppm orless of N, prepared through a finishing-annealing and a flattening step(including a substep of flattening annealing and a substep of applyingtension-coating), wherein the number of bendings in accordance with JISC 2550 is at least 6 over the transverse direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the percentage, relative to each orientedgrain, of a grain boundary of which the disorientation angle beforefinishing-annealing is 20-45°.

DETAILED DESCRIPTION

We employ a method for promoting secondary-recrystallization without aninhibitor.

As a result of diligent investigation on preferentialsecondary-recrystallization of Goss-oriented grain, we discovered that agrain boundary which has a disorientation angle of 20-45° in a primaryrecrystallization structure plays an important role and reported thisfinding in Acta Material, 45, 1285 (1997).

Specifically, we analyzed the primary recrystallized texture just beforesecondary-recrystallization of a grain-oriented electrical steel sheet,and studied the percentage (mass percent) of a grain boundary which hasa disorientation angle of 20-45° for each grain boundary around crystalgrains that have different crystal orientations. FIG. 1 shows theresults. The Euler space is expressed by a cross-section at Φ₂=45° ofEulerian angles (Φ₁,Φ,Φ₂). Major orientations including Goss orientationare illustrated.

FIG. 1 shows that the percentage of the grain boundary that has thedisorientation angle of 20-45° is highest at the Goss orientation.

Experimental data by C. G. Dunn et al. (AIME Transaction, 188, 368(1949)) suggested that the grain boundary that has a disorientationangle of 20-45° is a high-energy grain boundary. The high-energy grainboundary has a large area of free volume and disordered structure. Sincegrain boundary diffusion is a process in which atoms move through grainboundaries, it is faster in the high-energy grain boundary because ofits larger area of free volume.

Secondary-recrystallization in the conventional methods is known tooccur with diffusion-controlled growth and coarsening of a precipitateknown as an inhibitor. Considering these findings, we believe that theprecipitate on. the high-energy grain boundary grows preferentiallyduring the finishing-annealing, and thereby pinning of the grainboundary in the Goss orientation is preferentially removed to initiategrain boundary movement, and thus Goss-oriented grain grows.

We further developed this study and reached the following conclusion.

In summary, in the conventional methods, Goss-oriented grain in aprimary-recrystallized structure contains many high-energy grainboundaries, and the role of the inhibitor is to generate a difference inmobility between the high-energy grain boundary of Goss-oriented grainand other grain boundaries. Thus, if a difference in mobility isgenerated without using an inhibitor, it is possible to accumulate theGoss orientation during the secondary-recrystallization.

Initially, the high-energy grain boundary has a larger mobility thanother grain boundaries. However, since impurities in steel tend tosegregate at grain boundaries, particularly at the high-energy grainboundary, a large amount of impurities will reduce the difference inmobility between the high-energy grain boundary and other grainboundaries.

Accordingly, when materials are purified and the effects of impuritiesdescribed above are removed, the original difference in mobility due tothe grain boundary structure becomes obvious and Goss-oriented grain canbe developed preferentially during the secondary-recrystallization.

This is the principle of manufacturing an inhibitor-free steel sheet.

In the inhibitor-free steel sheet, the purification annealing is alsosometimes performed to remove residual impurities or to prepare, forexample, a forsterite film. As mentioned above, even in this case, itwas found that the bend properties may be deteriorated.

As a result of investigation as to the deterioration of the bendproperties in the inhibitor-free steel sheet, it was found that animmediate cause was a reduction in the grain boundary strengthassociated with precipitation of silicon nitrides at the grain boundary.

This precipitation of silicon nitrides at the grain boundary is partlycaused by nitrogen remaining in the steel after the purificationannealing. Theoretically, it may be possible to overcome this problem bysufficient purification annealing. However, nonuniform purification in acoil limits this possibility.

In the conventional manufacturing processes using S or Se as aninhibitor, the inhibitor in the steel retards the formation of a filmand thus nitrogen in the steel is easily purified. On the other hand, inthe inhibitor-free steel sheet, which originally contains fewerimpurities, a dense film is easily formed and therefore nitrogen in thesteel is difficult to remove. Accordingly, a new method for preventingsilicon nitrides from precipitating at the grain boundary is desired.

Further investigation of the coil showed that the bend properties weredeteriorated only at the transverse ends, even when the amounts ofnitrogen remaining at transverse ends and the transverse center of thecoil are similar. The term “end” of the coil used herein means an areabetween an endmost position and an inner position about 100 mm from theendmost position in the coil.

In other words, it might be possible to improve the bend properties bypreventing silicon nitrides from precipitating at the grain boundary,even when nitrogen in the steel is insufficiently removed. As a resultof diligent investigation, we have discovered that by controlling thehydrogen partial pressure depending on the annealing temperature duringthe purification annealing, precipitation of silicon nitrides at thegrain boundary can be prevented while nitrogen remains in the steel.

Although the reason the precipitation of silicon nitrides at the grainboundary is prevented is not clear, we believe the reason as follows:

Annealing of a steel sheet at high temperature in a hydrogen atmosphereinduces hydrogen attack, which embrittles a grain boundary of thesecondary-recrystallization grain; that is, microvoids or fissures areformed at the grain boundary. Since these microvoids or fissures haveexposed metal surfaces, silicon nitrides precipitate preferentially onthe exposed metal surface, that is, in microvoids or fissures of thegrain boundary when the temperature decreases during the purificationannealing. The involvement of hydrogen attack is supported by thefindings that a portion with deteriorated bend properties extends as ahydrogen attack promoter such as Sb increases in the steel.

In other words, purification annealing at high temperature and highhydrogen partial pressure enhances the grain boundary precipitation ofsilicon nitrides. Thus, the bend properties can be improved by avoidingthese conditions.

Each constituent feature of the method for manufacturing the electricalsteel sheet will be described below.

First, a material for the electrical steel sheet (typically, a steelslab) contains about 0.08 mass percent or less of carbon, about 2.0-8.0mass percent of Si, and about 0.005-3.0 mass percent of Mn, and alsocontains reduced amount of following elements; about 100 ppm or less ofAl, and about 50 ppm or less (mass ppm; the same shall applyhereinafter) each of N, S and Se.

Carbon content: about 0.08 mass percent or less

When the carbon content in the material exceeds about 0.08 mass percent,even if the material is subjected to decarburizing annealing, it becomesdifficult to decrease the carbon to about 50 ppm or less, at whichmagnetic aging can be avoided. Accordingly, the carbon content must beabout 0.08 mass percent or less. In terms of material properties, thecarbon content has no lower limit and may be substantially 0 masspercent. However, about 1 ppm is regarded as an industrial limit for thecarbon content.

Si content: about 2.0-8.0 mass percent

While Si increases the electrical resistance to improve iron losseffectively, such an effect cannot be sufficiently achieved with lessthan about 2.0 mass percent of Si. On the other hand, more than about8.0 mass percent of Si reduces workability. Thus, the Si content shouldbe about 2.0-8.0 mass percent.

Mn content: about 0.005-3.0 mass percent

While Mn is essential for improving hot-workability, such an effectcannot be sufficiently achieved with less than about 0.005 mass percentof Mn. On the other hand, more than about 3.0 mass percent of Mn reducesthe magnetic flux density. Thus, the Mn content should be about0.005-3.0 mass percent.

Al content: less than about 100 ppm; N, S, and Se contents: about 50 ppmor less each

To achieve satisfactory secondary-recrystallization, the content of Alimpurity should be less than about 100 ppm, and the content of S and Seimpurities should be about 50 ppm or less each. Preferably, the Alcontent is about 20-100 ppm. This lower limit is determined inconsideration of cost of reducing Al. Preferably, the contents of S andSe are about 45 ppm or less each.

Nitrogen content should be about 50 ppm or less to prevent the formationof silicon nitrides during the purification annealing. Preferably, thenitrogen content is about 50 ppm or less.

While lesser contents of these impurities are more preferred and thusmay be 0 ppm, the industrial limit of reducing them is about 1 ppm.

Advantageously, other nitride-forming elements, such as Ti, Nb, B, Ta,and V are each reduced to about 50 ppm or less to prevent thedeterioration of iron loss and to ensure excellent workability.Preferably, the Ti content is 20 ppm or less.

In addition to these essential elements and elements to be reduced, thefollowing elements may be used as appropriate in the present invention.

The material may contain about 0.005-1.50 mass percent of Ni and/orabout 0.01-1.50 mass percent of Cu to improve the hot-rolled sheetstructure and the magnetic properties. Amounts of Ni and/or Cu below therespective lower limits will not improve the magnetic propertiessignificantly, and amounts of Ni and/or Cu above the respective upperlimits will result in unstable secondary-recrystallization and adeterioration in magnetic properties.

Furthermore, the material may contain a total of 0.0050-0.50 masspercent of As, Te, Sb, Sn, P, Bi, Hg, Pb, Zn, and/or Cd to improve theiron loss. Alternatively, the material may contain a total of0.0050-0.50 mass percent of at least one of Cr, As, Te, Sb, Sn, P, Bi,Hg, Pb, Zn, and Cd. These elements at amounts below the lower limit intotal will not improve the iron loss significantly, and at amounts abovethe upper limit will suppress the growth of secondary-recrystallizationgrain.

Preferably, the remainder of the material is iron and inevitableimpurities. The inevitable impurities include the impurities describedabove and oxygen. The oxygen content is preferably about 40 ppm or less.

Then, molten steel that is adjusted to the optimum composition asdescribed above is smelted in a converter, an electric furnace, or thelike by conventional methods, is treated, for example, in vacuum ifdesired, and is processed by common ingot-making or continuous castinginto a slab (a steel slab), or by direct casting into a thin slab with athickness of about 100 mm or less.

The slab may be heated by conventional methods and hot-rolled, oralternatively, it may be hot-rolled immediately after casting withoutheating. The thin slab may be hot-rolled or may be subjected to thesubsequent steps without hot-rolling.

Preferably, the temperature of the slab before hot-rolling is about1250° C. or less to reduce scale during the hot-rolling. Furthermore,the slab is desirably heated to a lower temperature to eliminate harmfuleffects caused by the formation of a fine-grained crystal structure andby the contamination of inhibitor-forming components inevitably mixedinto the slab, and to achieve a primary-recrystallization structure ofuniform and sized grain. On the other hand, in view of the load on ahot-rolling line, the slab is usually heated to at least about 1000° C.Thus, the slab is preferably heated to about 1100-1250° C.

Then, annealing of the hot-rolled sheet is performed if desired; forexample, the annealing allows a Goss structure in the final sheetproduct to develop highly.

Preferably, the annealing temperature of the hot-rolled sheet is about800-1100° C. to achieve this effect. When the annealing temperature isless than about 800° C., a band structure during the hot rolling remainsand thus the uniform and sized grain level in the primary-recrystallizedstructure is reduced. This causes insufficient growth insecondary-recrystallization. On the other hand, when the annealingtemperature of the hot-rolled sheet exceeds about 1100° C., the grainsize after the annealing will increase. This is not preferable in termsof achieving a uniform and sized grain in the primary recrystallizationstructure. More preferably, the temperature of the hot-rolled sheet isabout 900-1100° C.

Cold-rolling is performed after the hot-rolling or the annealing of thehot-rolled sheet. The cold-rolling may be performed one time, or atleast two times if desired. When the cold-rolling is performed more thanonce, intermediate annealing is typically performed between eachcold-rolling. The conditions of the intermediate annealing may be inaccordance with conventional methods. In a conventional process using aslab as a starting material, a cold-rolled steel sheet is strip-shaped.

In the cold-rolling, a rolling temperature of about 100-300° C. and/orone or more aging treatments at about 100-300° C. during thecold-rolling is advantageous to develop a Goss structure.

After the cold-rolling, decarburizing annealing is performed, ifdesired, to reduce the carbon content to about 50 ppm or less,preferably about 30 ppm or less, at which magnetic aging no longeroccurs.

Preferably, the decarburizing annealing is performed at about 700-1000°C. in a wet atmosphere.

In addition, siliconization may be applied between the cold-rolling andsecondary-recrystallization annealing to increase the Si content.Conveniently, siliconization is applied after decarburizing annealing.

Then, a MgO-based annealing separator is applied to the sheet, andfinishing-annealing including secondary-recrystallization annealing andpurification annealing is performed to develop asecondary-recrystallization structure and a forsterite film. Preferably,MgO is at least about 80 mass percent of the annealing separator.

Alternatively, another annealing separator based on an element otherthan MgO is used, if desired, to generate a non-forsterite film.Examples of such an annealing separator include those based on Al₂O₃ orSiO₂. Annealing separators may be omitted if desired.

Advantageously, secondary-recrystallization annealing is performed atabout 800° C. or more on set of secondary-recrystallization. Since theheating rate to 800° C. does not significantly affect the magneticproperties, it may be determined arbitrarily. Preferably, thesecondary-recrystallization annealing is performed at about 1050° C. orless. Particularly when soaking is performed, the temperature of thesecondary-recrystallization annealing is preferably about 900° C. orless.

Preferably, the secondary-recrystallization annealing is performed for10 hours or more in the temperature range described above. Thus,typically in finishing-annealing, a cold-rolled steel strip is wound ina coil and is subjected to batch annealing.

In the subsequent purification annealing, the annealing temperature ispreferably about 1050° C. or more to generate a satisfactory forsteritefilm. An upper limit of the annealing temperature is about 1300° C. inview of cost. Preferably, the purification annealing is performed for1-20 hours.

Furthermore, controlling the annealing atmosphere is important in thepurification annealing to prevent deterioration in bend properties asfollows:

-   -   for purification annealing temperatures of 1170° C. or less,        adjust the hydrogen partial pressure in the atmosphere to about        0.8 atm or less in a temperature range of 1050° C. or more; and    -   for purification annealing temperatures above 1170° C., adjust        the hydrogen partial pressure in the atmosphere to about 0.4 atm        or less in a temperature range above 1170° C.

When the hydrogen partial pressure exceeds about 0.8 atm in atemperature range of 1170° C. or less in the former, or exceeds about0.4 atm in a temperature range above 1170° C. in the latter, voids willbe formed at a grain boundary by hydrogen attack in transverse ends,which are highly sensitive to the atmosphere. Then, N₂ that is dissolvedin the steel precipitates as silicon nitrides on the voids duringcooling causing a deterioration of the bend properties. Accordingly, byproviding an atmosphere having the hydrogen partial pressure definedabove to at least transverse ends of the coil, deteriorations in thebend properties can be prevented.

When the purification annealing temperature is above 1170° C., theeffect of atmosphere at 1050-1170° C. is relatively small, and thusthere is no need to control the hydrogen concentration in thistemperature range.

In view of avoiding explosion, the total pressure in an annealingfurnace during purification annealing is preferably 1.0 atm or more.Preferably, the gas used to adjust the hydrogen partial pressure is aninert gas, such as Ar, Ne, and He. Nitrogen may also be used, but is notpreferred because it may interfere with nitrogen removal from the steel.Thus, nitrogen is preferably less than 50%, more preferably less than30%, still more preferably 15% or less, and most preferablysubstantially 0% by volume.

As described above, the steel may contain at least one of Cr, As, Te,Sb, Sn, P, Bi, Hg, Pb, Zn, and Cd to improve iron loss. However, highcontents of these elements accelerate hydrogen attack. Thus, when thesteel contains about 0.0050 mass percent or more of these elements intotal, the conditions of the annealing atmosphere described above arepreferably replaced with the following conditions:

-   -   for purification annealing temperatures of 1170° C. or less,        adjust the hydrogen partial pressure in the atmosphere to about        0.6 atm or less in a temperature range of 1050° C. or more; and    -   for purification annealing temperatures above 1170° C., adjust        the hydrogen partial pressure in the atmosphere to about 0.2 atm        or less in a temperature range above 1170° C.

When these elements that accelerate hydrogen attack exceed about 0.5mass percent in total, the bend properties will not be improved.Therefore, these elements should be 0.5 mass percent or less.

As described above, secondary-recrystallization annealing andpurification annealing are typically performed sequentially and aretogether referred to as finishing-annealing. Theoretically,secondary-recrystallization annealing and purification annealing may beperformed independently in this order. In this case, an annealingseparator may be applied before either annealing process.

After purification annealing, flattening annealing is performed, ifdesired, for shape correction. Advantageously, an insulating coatingthat generates tension on the surface of the steel sheet is furtherapplied to improve iron loss. The flattening annealing, thetension-coating step, and their associated steps are herein referred toas a flattening step as a whole.

When finishing-annealing is performed on the coil in batch annealing, anelectrical steel sheet exhibits excellent bend properties over thetransverse direction of the coil. In other words, the bend propertiesafter finishing-annealing are not deteriorated over the transverse ends.Thus, the bend properties of the ends are excellent after thefinishing-annealing and the subsequent flattening step includingflattening annealing. In addition, the stability of manufacturing linein the flattening step and the subsequent steps is also excellent.

In the composition (excluding a film, such as a forsterite film) of theelectrical steel sheet, carbon is reduced to about 50 ppm or less, andS, Se, and Al are each reduced to about 15 ppm or less by purificationtreatment. Nitrogen is also reduced to about 35 ppm or less by thepurification treatment (a typical analytical limit is about 5 ppm).Other components are similar to those of the slab.

EXAMPLES Example 1

A steel slab that contained 0.050 mass percent of carbon, 3.25 masspercent of Si, 0.070 mass percent of Mn, 80 ppm of Al, 40 ppm of N, 20ppm of S, and 20 ppm of Se, and consisted essentially of iron andinevitable impurities, was heated to 1200° C. and was hot-rolled into acoiled sheet with a thickness of 2.2 mm. The hot-rolled sheet wasannealed at 1000° C. for 30 seconds, was subjected to removing scale onthe surface, and was cold-rolled with a tandem mill to a final thicknessof 0.28 mm. Then, the cold-rolled steel strip coil was degreased, wassubjected to decarburizing annealing at 840° C. for 120 seconds, wascoated with an annealing separator containing 90 mass percent of MgO and10 mass percent of TiO₂, and was subjected to batch finishing-annealingto produce final sheet products.

In the finishing-annealing, the sheets were subjected tosecondary-recrystallization annealing at 850° C. for about 50 hours, andwere subjected to subsequent purification annealing including heating at25° C./h to purification annealing temperatures shown in Table 1, andsoaking at the temperature for 5 hours. The hydrogen partial pressure inthe atmosphere was adjusted to values shown in Table 1 at temperaturesabove 1170° C. for purification annealing temperatures above 1170° C.,and at 1050° C. or more for the purification annealing at 1170° C. orless. The atmosphere had a total pressure of 1.0 atm and was balancedwith Ar.

Table 1 shows the magnetic properties (B₈: magnetic flux densities at amagnetizing force of 800 A/m) and bend properties of the resulting finalsheet products. The final sheet products contained less than 15 ppm ofcarbon, Al, S, or Se.

The magnetic properties were measured at a position where the bendproperties of the coils were evaluated. The bend properties weredetermined for a specimen 30 mm in width that was taken from atransverse end of the coil, specifically taken so that the center of thespecimen being at a position 45 mm inside from an endmost portion, inaccordance with a JIS C 2550 repeated bending test. A specimen thatformed a crack within 5 times of bending was determined to be defective(The same applies to the following examples). Likewise, when the bendproperties were also examined in the transverse center portions of thecoils, the results were all excellent (not shown).

TABLE 1 Hydrogen Residual Purification partial nitrogen Magneticannealing pressure content Bend properties No. temperature (° C.) (atm)(ppm) properties B₈(T) Remarks 1 1160 0 30 Good 1.89 This invention 21160 0.2 32 Good 1.90 This invention 3 1160 0.4 31 Good 1.90 Thisinvention 4 1160 0.6 33 Good 1.89 This invention 5 1160 0.8 29 Good 1.91This invention 6 1160 1.0 30 Poor 1.90 Comparative example 7 1170 0 28Good 1.90 This invention 8 1170 0.2 25 Good 1.89 This invention 9 11700.4 29 Good 1.90 This invention 10 1170 0.6 33 Good 1.89 This invention11 1170 0.8 30 Good 1.91 This invention 12 1170 1.0 32 Poor 1.90Comparative example 13 1180 0 28 Good 1.90 This invention 14 1180 0.2 26Good 1.89 This invention 15 1180 0.4 26 Good 1.90 This invention 16 11800.6 27 Poor 1.90 Comparative example 17 1180 0.8 29 Poor 1.89Comparative example 18 1180 1.0 26 Poor 1.91 Comparative example

Table 1 shows that the specimens that meet our conditions exhibitexcellent bend properties even at the transverse ends of the coils.

Example 2

Steel slabs that contained components shown in Tables 2-1 and 2-2, weresubstantially free of Se, and consisted essentially of iron andinevitable impurities as the remainder, were heated to 1200° C. and werehot-rolled into coiled sheets with a thickness of 2.2 mm. Thesehot-rolled sheets were annealed at 1000° C. for 30 seconds, weresubjected to removing scale on the surface, were cold-rolled with atandem mill to a final thickness of 0.28 mm, and were degreased. Then,the cold-rolled steel strips other than No. 42 steel were subjected todecarburizing annealing at 840° C. for 120 seconds. The steel stripswere coated with an annealing separator containing 90 mass percent ofMgO and 10 mass percent of TiO₂ (for No. 43 steel, an annealingseparator consisting of Al₂O₃ was applied), and were subjected to batchfinishing-annealing to produce final sheet products.

In the finishing-annealing, the strips were heated at 25° C./h fromsecondary-recrystallization annealing (850° C. for about 50 hours) totemperatures shown in Tables 2-1 and 2-2, and were subjected to thesubsequent purification annealing at the temperature for 5 hours. Thehydrogen partial pressure in the atmosphere was adjusted to values shownin Tables 2-1 and 2-2 at temperatures above 1170° C. for purificationannealing temperatures above 1170° C., and at 1050° C. or more for thepurification annealing at 1170° C. or less. The atmosphere had a totalpressure of 1.0 atm and was balanced with Ar. However, the totalpressure was 1.1 atm for No. 44 steel, and the balance gas was Ar and10% by volume of nitrogen for No. 45 steel.

Tables 2-1 and 2-2 show the magnetic properties and bend properties ofthe resulting final sheet products. The final sheet products containedless than 15 ppm of carbon (other than No. 42 steel), Al, S, Se, or N.

Like example 1, Tables 2-1 and 2-2 show the bend properties of the coilsat transverse ends. The bend properties at the transverse centerportions of the coils were all excellent.

TABLE 2-1 Purification Hydrogen annealing partial Magnetic Si Mn sol.AlN S Sb temperature pressure Bend properties No. C (mass %) (mass %)(mass %) (ppm) (ppm) (ppm) (mass %) (° C.) (atm) properties B₈(T)Remarks 1 0.04 3.25 0.07 50 50 20 0.002 1180 0 Good 1.90 This invention2 0.04 3.25 0.07 55 49 20 0.002 1180 0.2 Good 1.90 This invention 3 0.043.25 0.07 50 50 20 0.002 1180 0.4 Good 1.91 This invention 4 0.04 3.250.07 50 50 20 0.002 1180 0.6 Poor 1.88 Comparative example 5 0.04 3.250.07 48 50 20 0.002 1180 0.8 Poor 1.89 Comparative example 6 0.04 3.250.07 50 50 20 0.002 1180 1.0 Poor 1.89 Comparative example 7 0.04 3.250.07 47 50 20 0.002 1160 0 Good 1.90 This invention 8 0.04 3.25 0.07 5050 20 0.002 1160 0.2 Good 1.91 This invention 9 0.04 3.25 0.07 53 50 200.002 1160 0.4 Good 1.89 This invention 10 0.04 3.25 0.07 50 50 20 0.0021160 0.6 Good 1.90 This invention 11 0.04 3.25 0.07 52 50 20 0.002 11600.8 Good 1.88 This invention 12 0.04 3.25 0.07 50 50 20 0.002 1160 1.0Poor 1.90 Comparative example 13 0.04 3.25 0.07 47 50 20 0.005 1180 0Good 1.89 This invention 14 0.04 3.25 0.07 50 50 20 0.005 1180 0.2 Good1.91 This invention 15 0.04 3.25 0.07 53 50 20 0.005 1180 0.4 Poor 1.90Comparative example 16 0.04 3.25 0.07 50 50 20 0.005 1180 0.6 Poor 1.90Comparative example 17 0.04 3.25 0.07 53 50 20 0.005 1180 0.8 Poor 1.91Comparative example 18 0.04 3.25 0.07 50 50 20 0.005 1180 1.0 Poor 1.89Comparative example 19 0.04 3.25 0.07 47 50 20 0.050 1160 0 Good 1.90This invention 20 0.04 3.25 0.07 53 50 20 0.050 1160 0.2 Good 1.88 Thisinvention 21 0.04 3.25 0.07 50 50 20 0.050 1160 0.4 Good 1.90 Thisinvention 22 0.04 3.25 0.07 47 50 20 0.050 1160 0.6 Good 1.89 Thisinvention 23 0.04 3.25 0.07 50 50 20 0.050 1160 0.8 Poor 1.90Comparative example 24 0.04 3.25 0.07 53 50 20 0.050 1160 1.0 Poor 1.88Comparative example

TABLE 2-2 Purification Hydrogen annealing partial Magnetic Si Mn sol.AlN S Sb temperature pressure Bend properties No. C (mass %) (mass %)(mass %) (ppm) (ppm) (ppm) (mass %) (° C.) (atm) properties B₈(T)Remarks 25 0.04 3.25 0.07 50 50 20 0.050 1180 0 Good 1.90 This invention26 0.04 3.25 0.07 50 50 20 0.050 1180 0.2 Good 1.88 This invention 270.04 3.25 0.07 47 50 20 0.050 1180 0.4 Poor 1.88 Comparative example 280.04 3.25 0.07 50 50 20 0.050 1180 0.6 Poor 1.89 Comparative example 290.04 3.25 0.07 53 50 20 0.050 1180 0.8 Poor 1.88 Comparative example 300.04 3.25 0.07 50 50 20 0.050 1180 1.0 Poor 1.88 Comparative example 310.04 2.10 0.07 60 45 20 <0.001 1180 0 Good 1.90 This invention 32 0.047.80 0.07 60 45 20 <0.001 1180 0 Good 1.86 This invention 33 0.04 3.250.01 90 35 20 <0.001 1180 0 Good 1.91 This invention 34 0.04 3.25 2.5590 35 20 <0.001 1180 0 Good 1.90 This invention 35 0.03 3.00 0.05 65 2520 <0.001 1060 0.8 Good 1.89 This invention 36 0.05 3.50 0.10 65 25 20<0.001 1200 0.2 Good 1.88 This invention 37 0.04 3.25 0.07 92 45 20<0.001 1180 0 Good 1.90 This invention 38 0.04 3.25 0.07 80 23 20 <0.0011180 0 Good 1.90 This invention 39 0.04 3.00 0.07 70 30 40 <0.001 1180 0Good 1.88 This invention 40 0.04 3.00 0.07 70 30 50 <0.001 1180 0 Good1.87 This invention 41 0.07 3.00 0.07 85 35 20 <0.001 1180 0 Good 1.90This invention 42 0.003 3.00 0.07 80 40 20 <0.001 1180 0 Good 1.89 Thisinvention 43 0.04 3.25 0.07 60 30 20 <0.001 1180 0.2 Good 1.90 Thisinvention 44 0.03 3.25 0.06 75 45 20 <0.001 1180 0.2 Good 1.89 Thisinvention 45 0.03 3.50 0.06 75 50 20 <0.001 1180 0.4 Good 1.89 Thisinvention 46 0.03 3.25 0.06 75 40 20 <0.001 1180 0.4 Good 1.89 Thisinvention 47 0.03 3.25 0.06 75 45 20 <0.001 1180 0.3 Good 1.88 Thisinvention

Tables 2-1 and 2-2 show that the specimens that meet our conditionsexhibit excellent bend properties even at the transverse ends of thecoils. In particular, when 0.005 mass percent or more of Sb iscontained, hydrogen in purification annealing is preferably limited to alower level.

Example 3

Steel slabs that contained components shown in Table 3, weresubstantially free of Se, and consisted essentially of iron andinevitable impurities, were heated to 1200° C. and were hot-rolled intocoiled sheets with a thickness of 2.2 mm. These hot-rolled sheets wereannealed at 1000° C. for 30 seconds, were subjected to removing scale onthe surface, were cold-rolled with a tandem mill to a final thickness of0.28 mm. Then, the cold-rolled steel strip coils were degreased, weresubjected to decarburizing annealing at 840° C. for 120 seconds, werecoated with an annealing separator containing 90 mass percent of MgO and10 mass percent of TiO₂, and were subjected to batch finishing-annealingto produce final sheet products.

In the finishing-annealing, the sheets were subjected tosecondary-recrystallization annealing at 850° C. for about 50 hours, andwere subjected to the purification annealing including subsequentheating at 25° C./h to 1160° C., and subsequent soaking at 1160° C. for5 hours. The hydrogen partial pressure at 1050° C. or more was changedfrom 0 to 0.1 atm (total pressure: 1.0 atm) as shown in Table 3. Thebalance gas was Ar.

Table 3 shows the magnetic properties and bend properties of theresulting final sheet products. The final sheet products contained lessthan 15 ppm of carbon, Al, S, Se, or N.

Like example 1, Table 3 shows the bend properties of the coils attransverse ends. The bend properties at the transverse center portionsof the coils were all excellent.

TABLE 3 Other Hydrogen C Si Sb P Cr Bi compo- partial Bend Magnetic(mass (mass Mn sol.Al N S (mass (mass (mass (mass nents pressure proper-properties No. %) %) (mass %) (ppm) (ppm) (ppm) %) %) %) %) (mass %)(atm) ties B₈(T) Remarks 1 0.04 3.25 0.07 50 50 20 0.02 — — — — 0.2 Good1.90 This invention 2 0.04 3.25 0.07 55 50 20 0.02 — — — — 0.8 Poor 1.90Comparative example 3 0.04 3.25 0.07 50 50 20 — 0.02 — — — 0.2 Good 1.91This invention 4 0.04 3.25 0.07 50 50 20 — 0.02 — — — 1.0 Poor 1.88Comparative example 5 0.04 3.25 0.07 48 50 20 0.02 — 0.02 — — 0.6 Good1.89 This invention 6 0.04 3.25 0.07 50 50 20 — — — 0.03 — 0.2 Good 1.89This invention 7 0.04 3.25 0.07 47 50 20 — — — 0.03 — 1.0 Poor 1.90Comparative example 8 0.04 3.25 0.07 50 50 20 — 0.30 — — — 0.2 Good 1.91This invention 9 0.04 3.25 0.07 53 50 20 0.40 0.20 — — — 0.4 Poor 1.89Comparative example 10 0.04 3.25 0.07 50 49 20 — — — 0.60 — 0.6 Poor1.90 Comparative example 11 0.04 3.25 0.07 52 50 20 0.20 0.30 — — — 0.8Poor 1.88 Comparative example 12 0.03 3.20 0.09 60 47 30 — — — — As:0.01, 0.6 Good 1.88 This Te: 0.02, invention Hg: 0.01 13 0.05 3.30 0.0558 43 25 — — — — Pb: 0.01, 0.6 Good 1.88 This Zn: 0.01, invention Cd:0.02 14 0.04 3.30 0.07 60 30 20 0.03 — — — Ni: 0.1 0.2 Good 1.90 Thisinvention 15 0.04 3.30 0.07 65 30 20 0.03 — — — Cu: 0.2 0.2 Good 1.89This invention 16 0.04 3.30 0.07 70 30 20 0.03 — — — Ni: 0.7, 0.2 Good1.90 This Cu: 0.2 invention 17 0.04 3.30 0.07 80 45 20 — — — — Sn: 0.40.2 Good 1.89 This invention 18 0.04 3.30 0.07 70 40 20 — — — — Sn: 0.10.2 Good 1.89 This invention 19 0.04 3.30 0.07 90 45 20 — — — — Sn: 0.050.2 Good 1.90 This invention

Table 3 shows that the specimens that meet our conditions exhibitexcellent bend properties.

Example 4

A steel slab that had the same composition as that in EXAMPLE 1 washeated to 1200° C. and was hot-rolled into a coiled sheet with athickness of 2.4 mm. This hot-rolled sheet was not annealed and thescale on the surface was removed. The sheet was cold-rolled with atandem mill to a final thickness of 0.28 mm.

The cold-rolling was performed in two stages: the sheet was first rolledat 80° C. to 1.6 mm thickness followed by intermediate annealing at1000° C. for 60 seconds, and was then rolled at 200° C.

Then, the sheet was degreased, was subjected to decarburizing annealingat 840° C. for 120 seconds, was coated with a MgO-based annealingseparator, and was subjected to finishing-annealing to produce a finalsheet product.

In the finishing-annealing, the sheet was heated at 12.5° C./h from atleast 900° C. to 1160° C. and was held at 1160° C. for 5 hours. The heattreatment (i.e. heating) between about 900° C. and about 1050° C.corresponds to secondary-recrystallization annealing, and the subsequentheat treatment (i.e. heating and soaking) corresponds to purificationannealing. In the annealing, a hydrogen partial pressure at 1050° C. ormore was 0.6 atm (total pressure: 1.0 atm). The final sheet productcontained less than 15 ppm of carbon, Al, S, Se, or N.

The bend properties of the resulting steel sheet at a transverse end andat a transverse center portion of the coil were both excellent. Themagnetic flux density B₈ was 1.87 T.

INDUSTRIAL APPLICABILITY

Bend properties of, in particular, a final sheet product of agrain-oriented electrical steel sheet manufactured without using aninhibitor are improved. Thus, a grain-oriented electrical steel sheetwith excellent film properties can be consistently provided.

1. A method for manufacturing a grain-oriented electrical steel sheet,comprising the steps of: rolling a steel slab containing 0.08 masspercent or less of carbon, 2.0-8.0 mass percent of Si, and 0.005-3.0mass percent of Mn into a cold-rolled steel sheet; subsequentlyperforming decarburizing annealing of the cold-rolled steel sheet ifdesired; subsequently applying an annealing separator to the cold-rolledsteel sheet if desired; performing secondary-recrystallization annealingof the cold-rolled steel sheet; and subsequently performing purificationannealing of the cold-rolled steel sheet, wherein the steel slabcontains less than 100 ppm of Al and not more than 50 ppm each of N, S,and Se and the remainder being Fe and inevitable impurities, thepurification annealing is performed at 1050° C. or more, and the partialpressure of hydrogen in the atmosphere is adjusted to 0.4 atm or less ina temperature range above 1170° C. for a purification annealingconducted at a temperature above 1170° C., or 0.8 atm or less in atemperature range of 1050° C. or more for a purification annealingconducted at a temperature of 1170° C. or less.
 2. A method formanufacturing a grain-oriented electrical steel sheet, comprising thesteps of: rolling a steel slab containing 0.08 mass percent or less ofcarbon, 2.0-8.0 mass percent of Si, and 0.005-3.0 mass percent of Mn,and further containing 0.005-1.50mass percent of Ni and/or 0.01-1.50mass percent of Cu, into a cold-rolled steel sheet; subsequentlyperforming decarburizing annealing of the cold-rolled steel sheet ifdesired; subsequently applying an annealing separator to the cold-rolledsteel sheet if desired; performing secondary-recrystallization annealingof the cold-rolled steel sheet; and subsequently performing purificationannealing of the cold-rolled steel sheet, wherein the steel slabcontains less than 100 ppm of Al and not more than 50 ppm each of N, S,and Se and the remainder being Fe and inevitable impurities, thepurification annealing is performed at 1050° C. or more, and the partialpressure of hydrogen in the atmosphere is adjusted to 0.4 atm or less ina temperature range above 1170° C. for a purification annealingconducted at a temperature above 1170° C., or 0.8 atm or less in atemperature range of 1050° C., or more for a purification annealingconducted at a temperature of 1170° C. or less.
 3. A method formanufacturing a grain-oriented electrical steel sheet, comprising thesteps of: rolling a steel slab containing 0.08 mass percent or less ofcarbon, 2.0-8.0 mass percent of Si, and 0.005-3.0 mass percent of Mn,and further containing a total of 0.0050-0.50 mass percent of at leastone of Cr, As, Te, Sb, Sn, P, Bi, Hg, Pb, Zn, and Cd, into a cold-rolledsteel sheet; subsequently performing decarburizing annealing of thecold-rolled steel sheet if desired; subsequently applying an annealingseparator to the cold-rolled steel sheet if desired; performingsecondary-recrystallization annealing of the cold-rolled steel sheet;and subsequently performing purification annealing of the cold-rolledsteel sheet, wherein the steel slab contains less than 100 ppm of Al andnot more than 50 ppm each of N, S, and Se, the purification annealing isperformed at 1050° C. or more, and the partial pressure of the hydrogenin the atmosphere is adjusted to 0.2 atm or less in a temperature rangeabove 1170° C. for a purification annealing conducted at a temperatureabove 1170° C., or 0.6 atm or less in a temperature range of 1050° C. ormore for a purification annealing conducted at a temperature of 1170° C.or less.
 4. A method for manufacturing a grain-oriented electrical steelsheet, comprising the steps of: rolling a steel slab containing 0.08mass percent or less of carbon, 2.0-8.0 mass percent of Si, and0.005-3.0 mass percent of Mn, and further containing a total of0.0050-0.50 mass percent of at least one of As, Te, Sb, Sn, P, Bi, Hg,Pb, Zn, and Cd, into a cold-rolled steel sheet; subsequently performingdecarburizing annealing of the cold-rolled steel sheet if desired;subsequently applying an annealing separator to the cold-rolled steelsheet if desired; performing secondary-recrystallization annealing ofthe cold-rolled steel sheet; and subsequently performing purificationannealing of the cold-rolled steel sheet, wherein the steel slabcontains less than 100 ppm of Al and not more than 50 ppm each of N, S,and Se, the purification annealing is performed at 1050° C. or more, andthe partial pressure of the hydrogen atmosphere is adjusted to 0.2 atmor less in a temperature range above 1170° C. for a purificationannealing conducted at a temperature above 1170° C., or 0.6 atm or lessin a temperature range of 1050° C. or more for a purification annealingconducted at a temperature of 1170° C. or less.
 5. A method formanufacturing a grain-oriented electrical steel sheet, comprising thesteps of: rolling a steel slab containing 0.08 mass percent or less ofcarbon, 2.0-8.0 mass percent of Si, and 0.005-3.0 mass percent of Mn,and further containing 0.005-1.50 mass percent of Ni and/or 0.01-1.50mass percent of Cu, and a total of 0.0050-0.50 mass percent of at leastone of Cr, As, Te, Sb, Sn, P, Bi, Hq, Pb, Zn, and Cd, into a cold-rolledsteel sheet; subsequently performing decarburizing annealing of thecold-rolled steel sheet if desired; subsequently applying an annealingseparator to the cold-rolled steel sheet if desired; performingsecondary-recrystallization annealing of the cold-rolled steel sheet;and subsequently performing purification annealing of the cold-rolledsteel sheet, wherein the steel slab contains less than 100 ppm of Al andnot more than 50 ppm each of N, S, and Se, the purification annealing isperformed at 1050° C. or more, and the partial pressure of hydrogen inthe atmosphere is adjusted to 0.2 atm or less in a temperature rangeabove 1170° C. for a purification annealing conducted at a temperatureabove 1170° C., or 0.6 atm or less in a temperature range of 1050° C. ormore for a purification annealing conducted at a temperature of 1170° C.or less.
 6. The method for manufacturing a grain-oriented electricalsteel sheet according to any one of claims 1 to 4 and 5, wherein, as theannealing separator, a MgO-based annealing separator is applied to thecold-rolled steel sheet.
 7. The method for manufacturing agrain-oriented electrical steel sheet according to any one of claims 1to 4 and 5, wherein the rolling step comprises the substeps of:hot-rolling the steel slab; annealing the hot-rolled steel sheet ifdesired; and performing cold-rolling one time, or at least two timeswith intermediate annealing therebetween to produce the cold-rolledsteel sheet.
 8. The method for manufacturing a grain-oriented electricalsteel sheet according to any one of claims 1 to 4 and 5, wherein thenitrogen content in the atmosphere is less than 50% by volume in thepurification annealing.
 9. The method for manufacturing a grain-orientedelectrical steel sheet according to any one of claims 1 to 4 and 5,wherein the rolling comprises a cold-rolling substep of preparing acold-rolled steel strip, and the cold-rolled steel strip is subjected tothe secondary-recrystallization annealing and the purification annealingto produce a strip-shaped grain-oriented electrical steel sheet.